Cap layers including aluminum nitride for nitride-based transistors

ABSTRACT

High electron mobility transistors are provided that include a non-uniform aluminum concentration AlGaN based cap layer having a high aluminum concentration adjacent a surface of the cap layer that is remote from the barrier layer on which the cap layer is provided. High electron mobility transistors are provided that include a cap layer having a doped region adjacent a surface of the cap layer that is remote from the barrier layer on which the cap layer is provided. Graphitic BN passivation structures for wide bandgap semiconductor devices are provided. SiC passivation structures for Group III-nitride semiconductor devices are provided. Oxygen anneals of passivation structures are also provided. Ohmic contacts without a recess are also provided.

CLAIM OF PRIORITY

This application is a continuation in part of and claims priority fromU.S. patent application Ser. No. 10/996,249, filed Nov. 23, 2004, nowU.S. Pat. No. 7,456,443 the content of which is hereby incorporatedherein by reference as if set forth in its entirety.

FIELD OF THE INVENTION

The present invention relates to semiconductor devices and, moreparticularly, to transistors that incorporate nitride-based activelayers.

BACKGROUND

Materials such as silicon (Si) and gallium arsenide (GaAs) have foundwide application in semiconductor devices for lower power and (in thecase of Si) lower frequency applications. These, more familiar,semiconductor materials may not be well suited for higher power and/orhigh frequency applications, however, because of their relatively smallbandgaps (e.g., 1.12 eV for Si and 1.42 for GaAs at room temperature)and/or relatively small breakdown voltages.

In light of the difficulties presented by Si and GaAs, interest in highpower, high temperature and/or high frequency applications and deviceshas turned to wide bandgap semiconductor materials such as siliconcarbide (2.996 eV for alpha SiC at room temperature) and the Group IIInitrides (e.g., 3.36 eV for GaN at room temperature). These materials,typically, have higher electric field breakdown strengths and higherelectron saturation velocities as compared to gallium arsenide andsilicon.

A device of particular interest for high power and/or high frequencyapplications is the High Electron Mobility Transistor (HEMT), which, incertain cases, is also known as a modulation doped field effecttransistor (MODFET). These devices may offer operational advantagesunder a number of circumstances because a two-dimensional electron gas(2DEG) is formed at the heterojunction of two semiconductor materialswith different bandgap energies, and where the smaller bandgap materialhas a higher electron affinity. The 2DEG is an accumulation layer in theundoped (“unintentionally doped”), smaller bandgap material and cancontain a very high sheet electron concentration in excess of, forexample, 10¹³ carriers/cm². Additionally, electrons that originate inthe wider-bandgap semiconductor transfer to the 2DEG, allowing a highelectron mobility due to reduced ionized impurity scattering.

This combination of high carrier concentration and high carrier mobilitycan give the HEMT a very large transconductance and may provide a strongperformance advantage over metal-semiconductor field effect transistors(MESFETs) for high-frequency applications.

High electron mobility transistors fabricated in the galliumnitride/aluminum gallium nitride (GaN/AlGaN) material system have thepotential to generate large amounts of RF power because of thecombination of material characteristics that includes the aforementionedhigh breakdown fields, their wide bandgaps, large conduction bandoffset, and/or high saturated electron drift velocity. A major portionof the electrons in the 2DEG is attributed to polarization in the AlGaN.HEMTs in the GaN/AlGaN system have already been demonstrated. U.S. Pat.Nos. 5,192,987 and 5,296,395 describe AlGaN/GaN HEMT structures andmethods of manufacture. U.S. Pat. No. 6,316,793, to Sheppard et al.,which is commonly assigned and is incorporated herein by reference,describes a HEMT device having a semi-insulating silicon carbidesubstrate, an aluminum nitride-buffer layer on the substrate, aninsulating gallium nitride layer on the buffer layer, an aluminumgallium nitride barrier layer on the gallium nitride layer, and apassivation layer on the aluminum gallium nitride active structure.

SUMMARY OF THE INVENTION

Some embodiments of the present invention provide Group III-nitride highelectron mobility transistors and methods of fabricating GroupIII-nitride high electron mobility transistors that include a GroupIII-nitride based channel layer, a Group III-nitride based barrier layeron the channel layer and a non-uniform composition AlGaN based cap layeron the barrier layer. The non-uniform composition AlGaN based cap layerhas a higher concentration of Al adjacent a surface of the cap layerthat is remote from the barrier layer than is present in a region withinthe AlGaN based cap layer. In particular embodiments of the presentinvention having a gate recessed through the cap layer, the higherconcentration of Al extends into the cap layer from about 30 Å to about1000 Å. In particular embodiments of the present invention having a gateon the cap layer, the higher concentration of Al extends into the caplayer from about 2.5 Å to about 100 Å.

In further embodiments of the present invention, the AlGaN based caplayer includes a first region of Al_(x)Ga_(1-x)N at the surface of thecap layer, where x≦1 and a second region of Al_(y)Ga_(1-y)N within theAlGaN based cap layer, where y<1 and y<x. The value of x may be fromabout 0.2 to about 1 and y is from about 0.15 to about 0.3. Inparticular embodiments of the present invention, the difference betweenx and y and/or the thickness of the cap layer may be selected to preventformation of a second 2DEG in the cap layer. In other embodiments of thepresent invention where the gate is recessed through the cap layer butdoes not touch the cap layer, the difference between x and y and/or thethickness of the cap layer may be selected to provide a second 2DEG inthe cap layer.

In additional embodiments of the present invention, the AlGaN based caplayer further includes a third region of Al_(z)Ga_(1-z)N at an interfacebetween the barrier layer and the AlGaN based cap layer, where z≦1 andz≠y. In some embodiments, z>y. In other embodiments, z>x. In stillfurther embodiments, z≦x.

In particular embodiments of the present invention, the channel layercomprises a GaN layer, the barrier layer comprises an AlGaN layer andthe cap layer comprises an AlGaN layer.

Some embodiments of the present invention provide Group III-nitride highelectron mobility transistors and methods of fabricating GroupIII-nitride high electron mobility transistors that include a GroupIII-nitride based channel layer, a Group III-nitride based barrier layeron the channel layer and a GaN based cap layer on the barrier layer. TheGaN based cap layer has a doped region adjacent a surface of the caplayer and is remote from the barrier layer.

In certain embodiments, the doped region is a region doped with n-typedopants. In particular embodiments of the present invention without agate recess, the doped region extends into the cap layer from about 2.5Å to about 50 Å. In particular embodiments of the present invention witha gate recess, the doped region extends into the cap layer from about 20Å to about 5000 Å. The doped region may provide a dopant concentrationof from about 10¹⁸ to about 10²¹ cm⁻³. The n-type dopants may be Si, Geor O. In particular embodiments of the present invention, the dopedregion may be one or more delta-doped regions at or near the surface ofthe cap layer and may, for example, have a dopant concentration of fromabout 10¹¹ to about 10¹⁵ cm⁻². In particular embodiments of the presentinvention, the dopant is O that extends into the cap layer about 20 Å.

In other embodiments, the doped region is a region doped with p-typedopants. In particular embodiments of the present invention without agate recess, the doped region extends into the cap layer from about 2.5Å to about 50 Å. In particular embodiments of the present invention witha gate recess, the doped region extends into the cap layer from about 30Å to about 5000 Å. The doped region may provide a dopant concentrationof from about 10¹⁶ to about 10²² cm⁻³. The p-type dopants may be Mg, Be,Zn, Ca or C. In particular embodiments of the present invention, thedoped region may be one or more delta-doped regions at or near thesurface of the cap layer and may, for example, have a dopantconcentration of from about 10¹¹ to about 10¹⁵ cm⁻².

In still further embodiments, the doped region is a region doped withdeep level dopants. In particular embodiments of the present inventionwithout a gate recess, the doped region extends into the cap layer fromabout 2.5 Å to about 100 Å. In particular embodiments of the presentinvention with a gate recess, the doped region extends into the caplayer from about 30 Å to about 5000 Å. The doped region may provide adopant concentration of from about 10¹⁶ to about 10²² cm⁻³. The deeplevel dopants may be Fe, C, V, Cr, Mn, Ni, Co or other rare earthelements.

In additional embodiments of the present invention, the doped region isa first doped region and the cap layer further includes a second dopedregion. The second doped region has a dopant concentration less than thedopant concentration of the first doped region. The second doped regionmay be the remainder of the cap layer not in the first doped region.

In particular embodiments, the channel layer comprises a GaN layer, thebarrier layer comprises an AlGaN layer and the cap layer comprises a GaNor an AlGaN layer.

Some embodiments of the present invention, provide methods forpassivating a surface of a wide bandgap semiconductor device thatinclude forming a graphitic and/or amorphous BN layer on a least aportion of a surface of a region of wide bandgap semiconductor materialof the wide-bandgap semiconductor device. Corresponding structures arealso provided.

In further embodiments of the present invention, the wide bandgapsemiconductor device is a Group III-nitride semiconductor device. Forexample, the wide bandgap semiconductor device may be a GaN basedsemiconductor device. Furthermore, the wide bandgap semiconductor devicemay be a Group III-nitride high electron mobility transistor.

In additional embodiments of the present invention, forming thegraphitic and/or amorphous BN layer is carried out at a temperature lessthan a decomposition temperature of wide bandgap semiconductor materialsin the wide bandgap semiconductor device. Forming the graphitic and/oramorphous BN layer may be carried out at a temperature less than about1100° C., in some embodiments at a temperature of less than about 1000°C. and in particular embodiments at a temperature of less than about900° C. Also, the BN layer may formed to be non-single crystal. Thegraphitic and/or amorphous BN layer may be formed to a thickness of fromabout 3 Å to about 1 μm.

Some embodiments of the present invention provide methods of passivatinga surface of a Group III-nitride semiconductor device by forming a SiClayer on a least a portion of a surface of a region of Group III-nitridesemiconductor material of the Group III-nitride semiconductor device.Corresponding structures are also provided.

In certain embodiments, the Group III-nitride semiconductor device maybe a GaN based semiconductor device. In further embodiments, the GroupIII-nitride semiconductor device may be a Group III-nitride highelectron mobility transistor.

In additional embodiments of the present invention, forming the SiClayer is carried out at a temperature less than a decompositiontemperature of Group III-nitride semiconductor materials in the GroupIII-nitride semiconductor device. For example, forming the SiC layer iscarried out at a temperature less than about 1100° C., in someembodiments at a temperature of less than about 1000° C. and inparticular embodiments at a temperature of less than about 900° C. Also,the SiC layer may be formed to be non-single crystal. In particularembodiments, forming the SiC layer comprises forming a 3C SiC layer. TheSiC layer may be formed to a thickness of from about 3 Å to about 1 μm.

Further embodiments of the present invention comprise methods ofproviding passivation structures for wide bandgap semiconductor devices,such as Group III-nitride semiconductor devices, comprising annealing apassivation layer directly on a Group III-nitride layer in an oxygencontaining environment. The passivation layer may be, for example, SiN,BN, MgN and/or SiC. In still other embodiments, the passivation layerincludes SiO₂, MgO, Al₂O₃, Sc₂O₃ and/or AlN.

The annealing may be carried out at a temperature of from about 100° C.to about 1000° C. and for a time of from about 10 seconds to about 1hour. The oxygen containing environment may be only oxygen, oxygen inN₂, oxygen in another inert gas, such as argon, oxygen in dry air, CO,CO₂, NO, NO₂ and/or ozone. The annealing may be performed at atemperature and for a time insufficient to oxidize the structureunderlying the passivation layer but sufficient to remove at least somehydrogen from the passivation layer. Some carbon may also be removedfrom the passivation layer.

Additional embodiments of the present invention provide methods offabricating a passivation structure for a Group III-nitridesemiconductor device by forming a passivation layer directly on a leasta portion of a surface of a region of Group III-nitride semiconductormaterial of the Group III-nitride semiconductor device and annealing thepassivation layer in D₂ and/or D₂O. In some embodiments, the passivationlayer includes SiN and/or MgN. In other embodiments, the passivationlayer includes BN and/or SiC. In still other embodiments, thepassivation layer includes SiO₂, MgO, Al₂O₃, Sc₂O₃ and/or AlN.

The annealing may be performed at a temperature and for a timeinsufficient to oxidize a structure underlying the passivation layer butsufficient to remove at least some hydrogen from the passivation layeror exchange some hydrogen with deuterium. Furthermore, the GroupIII-nitride semiconductor material may be a GaN based material.

Additional embodiments of the present invention provide GroupIII-nitride high electron mobility transistors and method of fabricatingGroup III-nitride high electron mobility transistors that include aGroup III-nitride based channel layer, a Group III-nitride based barrierlayer on the channel layer and an AlN cap layer on the barrier layer.The transistor may further include a gate contact recessed into the AlNcap layer. In such embodiments, the AlN cap layer has a thickness offrom about 5 to about 5000 Å. In some embodiments of the presentinvention, the AlN layer may not be coherent with the underlying layer,may be non-single crystalline, may be formed ex-situ and/or may beformed by a lower quality formation process, such as by PVD rather thanCVD. The transistor may also include a gate contact on the AlN cap layerand not recessed into the AlN cap layer. In such embodiments, the AlNcap layer has a thickness of from about 2 Å to about 20 Å. Additionally,the channel layer may be a GaN layer and the barrier layer may be anAlGaN layer.

Still further embodiments of the present invention provide GroupIII-nitride high electron mobility transistors and methods offabricating Group III-nitride high electron mobility transistors thatinclude a Group III-nitride based channel layer, a Group III-nitridebased barrier layer on the channel layer, a protective layer on thebarrier layer, a gate contact on the barrier layer and ohmic contacts onthe protective layer. In some embodiments of the present invention, theprotective layer includes SiN. In other embodiments, the protectivelayer includes BN or MgN. In further embodiments, the protective layercomprises multiple layers, such as a layer of SiN and a layer of AlN. Inparticular embodiments of the present invention, the protective layerhas a thickness of from about 1 Å to about 10 Å. In certain embodiments,the protective layer has a thickness of about one monolayer.

In still further embodiments of the present invention, the gate contactis on the protective layer. Also, the ohmic contacts may be directly onthe protective layer. The protective layer may be formed in-situ withforming the barrier layer.

Some embodiments of the present invention provide Group III-nitrideHEMTs including a Group III-nitride based channel layer and a GroupIII-nitride based barrier layer on the channel layer. A multilayer caplayer is provided on the barrier layer. The multilayer cap layerincludes a layer including aluminum nitride (AlN) on the barrier layerand a gallium nitride (GaN) layer on the layer including AlN. A SiNpassivation layer is provided on the GaN layer.

In further embodiments of the present invention, the layer including AlNmay include aluminum gallium nitride (AlGaN), aluminum indium nitride(AlInN) and/or AlN. The layer including AlN may have a thickness of fromabout 3 to about 30 Å.

In still further embodiments of the present invention, the GaN layer mayinclude an aluminum gallium nitride (AlGaN) layer having a lowmole-fraction. The mole-fraction of the AlGaN layer may be from aboutzero percent of a mole-fraction of the barrier layer to about themole-fraction of the barrier layer. The mole-fraction of the AlGaN layermay be greater than the mole-fraction of the barrier layer.

In some embodiments of the present invention, the layer including AlNmay include an AlGaN layer having a high mole-fraction.

Further embodiments of the present invention provide Group III-nitrideHEMTs including a Group III-nitride based channel layer and a GroupIII-nitride based barrier layer on the channel layer. A multilayer caplayer is provided on the barrier layer. The multilayer cap layerincludes an AlGaN layer on the barrier layer and an AlN layer on theAlGaN layer.

In still further embodiments of the present invention, the AlN layer mayhave a thickness of about 10 Å. The HEMT may further include a gatecontact that is not recessed into the multilayer cap layer. A thicknessof the AlGaN layer may be from about 5.0 to about 50.0 Å.

Some embodiments of the present invention provide Group III-nitrideHEMTs including a Group III-nitride based channel layer and a GroupIII-nitride based barrier layer on the channel layer. A multilayer caplayer is provided on the barrier layer. The multilayer cap layerincludes an AlGaN layer on the barrier layer, an AlN layer on the AlGaNlayer and a GaN layer on the AlN layer.

In further embodiments of the present invention, the AlN layer may havea thickness of about 10.0 Å and the GaN layer may have a thickness ofabout 20 Å. In certain embodiments of the present invention, the HEMThas a barrier of greater than about 3.0 V.

Various combinations and/or sub-combinations of cap layers, passivationlayers, protective layers and/or anneals of passivation layers may alsobe provided according to some embodiments of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are cross-sectional schematic drawings illustratingtransistors having a cap layer according to some embodiments of thepresent invention.

FIGS. 2A and 2B are cross-sectional schematic drawings illustratingtransistors having a cap layer according to some embodiments of thepresent invention.

FIGS. 3A and 3B are cross-sectional schematic drawings illustratinggraphitic and/or amorphous BN passivation layers according to someembodiments of the present invention.

FIGS. 4A and 4B are cross-sectional schematic drawings illustrating SiCpassivation layers according to some embodiments of the presentinvention.

FIGS. 5A and 5B are cross-sectional schematic drawings illustratingtransistors having a cap layer according to some embodiments of thepresent invention.

FIG. 6 is a cross-sectional schematic drawing illustrating transistorshaving ohmic contacts on a protective layer according to someembodiments of the present invention.

FIG. 7 is a cross-sectional schematic drawing illustrating transistorsincluding a protective layer and a cap layer according to someembodiments of the present invention.

FIG. 8 is a cross-sectional schematic drawing illustrating transistorshaving a multi-layer cap layer according to some embodiments of thepresent invention.

FIG. 9 is a cross-sectional schematic drawing illustrating transistorshaving a multi-layer cap layer according to some embodiments of thepresent invention.

FIG. 10 is a graph illustrating simulated conduction band edge andelectron density as a function of depth for devices having an AlGaN capaccording to some embodiments of the present invention.

FIG. 11 is a graph illustrating simulated conduction band edge andelectron density as a function of depth for devices having an AlN cap onAlGaN according to some embodiments of the present invention.

FIG. 12 is a graph illustrating simulated conduction band edge andelectron density as a function of depth for devices having a GaN/AlN capon AlGaN according to some embodiments of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, in which embodiments of theinvention are shown. However, this invention should not be construed aslimited to the embodiments set forth herein. Rather, these embodimentsare provided so that this disclosure will be thorough and complete, andwill fully convey the scope of the invention to those skilled in theart. In the drawings, the thickness of layers and regions areexaggerated for clarity. Like numbers refer to like elements throughout.As used herein the term “and/or” includes any and all combinations ofone or more of the associated listed items.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

It will be understood that when an element such as a layer, region orsubstrate is referred to as being “on” or extending “onto” anotherelement, it can be directly on or extend directly onto the other elementor intervening elements may also be present. In contrast, when anelement is referred to as being “directly on” or extending “directlyonto” another element, there are no intervening elements present. Itwill also be understood that when an element is referred to as being“connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directly coupled” to another element, there are nointervening elements present. Like numbers refer to like elementsthroughout the specification.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, components, regions, layersand/or sections, these elements, components, regions, layers and/orsections should not be limited by these terms. These terms are only usedto distinguish one element, component, region, layer or section fromanother region, layer or section. Thus, a first element, component,region, layer or section discussed below could be termed a secondelement, component, region, layer or section without departing from theteachings of the present invention.

Furthermore, relative terms, such as “lower” or “bottom” and “upper” or“top,” may be used herein to describe one element's relationship toanother elements as illustrated in the Figures. It will be understoodthat relative terms are intended to encompass different orientations ofthe device in addition to the orientation depicted in the Figures. Forexample, if the device in the Figures is turned over, elements describedas being on the “lower” side of other elements would then be oriented on“upper” sides of the other elements. The exemplary term “lower”, cantherefore, encompass both an orientation of “lower” and “upper,”depending of the particular orientation of the figure. Similarly, if thedevice in one of the figures is turned over, elements described as“below” or “beneath” other elements would then be oriented “above” theother elements. The exemplary terms “below” or “beneath” can, therefore,encompass both an orientation of above and below. Furthermore, the term“outer” may be used to refer to a surface and/or layer that is farthestaway from a substrate.

Embodiments of the present invention are described herein with referenceto cross-section illustrations that are schematic illustrations ofidealized embodiments of the present invention. As such, variations fromthe shapes of the illustrations as a result, for example, ofmanufacturing techniques and/or tolerances, are to be expected. Thus,embodiments of the present invention should not be construed as limitedto the particular shapes of regions illustrated herein but are toinclude deviations in shapes that result, for example, frommanufacturing. For example, an etched region illustrated as a rectanglewill, typically, have tapered, rounded or curved features. Thus, theregions illustrated in the figures are schematic in nature and theirshapes are not intended to illustrate the precise shape of a region of adevice and are not intended to limit the scope of the present invention.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this invention belongs. It will befurther understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art andwill not be interpreted in an idealized or overly formal sense unlessexpressly so defined herein.

It will also be appreciated by those of skill in the art that referencesto a structure or feature that is disposed “adjacent” another featuremay have portions that overlap or underlie the adjacent feature.

Embodiments of the present invention may be particularly well suited foruse in nitride-based devices such as Group III-nitride based HEMTs. Asused herein, the term “Group III nitride” refers to those semiconductingcompounds formed between nitrogen and the elements in Group III of theperiodic table, usually aluminum (Al), gallium (Ga), and/or indium (In).The term also refers to ternary and quaternary compounds such as AlGaNand AlInGaN. As is well understood by those in this art, the Group IIIelements can combine with nitrogen to form binary (e.g., GaN), ternary(e.g., AlGaN, AlInN), and quaternary (e.g., AlInGaN) compounds. Thesecompounds all have empirical formulas in which one mole of nitrogen iscombined with a total of one mole of the Group III elements.Accordingly, formulas such as Al_(x)Ga_(1-x)N where 0≦x≦1 are often usedto describe them.

Suitable structures and techniques for fabricating GaN-based HEMTs thatmay utilize embodiments of the present invention are described, forexample, in commonly assigned U.S. Pat. No. 6,316,793 and U.S. PatentPublication No. 2002/0066908A1 filed Jul. 12, 2001 and published Jun. 6,2002, for “ALUMINUM GALLIUM NITRIDE/GALLIUM NITRIDE HIGH ELECTRONMOBILITY TRANSISTORS HAVING A GATE CONTACT ON A GALLIUM NITRIDE BASEDCAP SEGMENT AND METHODS OF FABRICATING SAME,” U. S. Patent PublicationNo. 2002/0167023A1 to Smorchkova et al., published Nov. 14, 2002,entitled “GROUP-III NITRIDE BASED HIGH ELECTRON MOBILITY TRANSISTOR(HEMT) WITH BARRIER/SPACER LAYER”, U.S. patent application Ser. No.10/617,843 filed Jul. 11, 2003 for “NITRIDE-BASED TRANSISTORS ANDMETHODS OF FABRICATION THEREOF USING NON-ETCHED CONTACT RECESSES,” U.S.patent application Ser. No. 10/772,882 filed Feb. 5, 2004 for “NITRIDEHETEROJUNCTION TRANSISTORS HAVING CHARGE-TRANSFER INDUCED ENERGYBARRIERS AND METHODS OF FABRICATING THE SAME,” U.S. patent applicationSer. No. 10/897,726, filed Jul. 23, 2004 entitled “METHODS OFFABRICATING NITRIDE-BASED TRANSISTORS WITH A CAP LAYER AND A RECESSEDGATE,” U.S. patent application Ser. No. 10/849,617, filed May 20, 2004entitled “METHODS OF FABRICATING NITRIDE-BASED TRANSISTORS HAVINGREGROWN OHMIC CONTACT REGIONS AND NITRIDE-BASED TRANSISTORS HAVINGREGROWN OHMIC CONTACT REGIONS,” U.S. patent application Ser. No.10/849,589, filed May 20, 2004 and entitled “SEMICONDUCTOR DEVICESHAVING A HYBRID CHANNEL LAYER, CURRENT APERTURE TRANSISTORS AND METHODSOF FABRICATING SAME” and U.S. Patent Publication No. 2003/0020092 filedJul. 23, 2002 and published Jan. 30, 2003 for “INSULATING GATE ALGAN/GANHEMT”, the disclosures of which are hereby incorporated herein byreference in their entirety.

Some embodiments of the present invention provide nitride-based HEMTswith an AlGaN cap layer that has a higher concentration of AlGaN such asat a surface that is remote from the barrier layer than at other regionsof the AlGaN cap layer. Thus, the device may have a layer with a highconcentration of Al as an outer surface of the device. Such a layer mayimprove robustness of the device during processing and/or deviceoperation over a conventional device that includes a uniform Alconcentration or a reduced Al concentration at its outer surface. Forexample, the increased Al concentration at the surface may not besusceptible to etching or other chemical reactions at high temperaturesdue to the stronger Al—N bonds compared to Ga—N bonds.

In particular embodiments of the present invention, nitride-based HEMTswith an AlN cap layer on the barrier layer is provided. Thus, the devicemay have a layer with a high concentration of Al as an outer surface ofthe device that, as discussed above, may improve robustness of thedevice during processing and/or device operation over a conventionaldevice.

In further embodiments of the present invention, the outer surface ofthe cap layer of a nitride-based HEMT is doped with p-type, n-type ordeep-level dopants such that the cap layer has a higher concentration ofdopants at a surface of the cap layer that is remote from the barrierlayer than at other regions of the cap layer. The cap layer may be a GaNbased cap layer. The dopants at the outer surface of the device maysegregate to dislocations in the cap layer and, thereby, reduce gateleakage along the dislocations. The dopant may have differentcharacteristics when at a dislocation than when in the bulk crystal. Forexample, a shallow dopant in the bulk crystal may have characteristicsof a deep level when at a dislocation. Thus, references to p-type,n-type of deep-level dopants refers to the characteristics of thedopants in the bulk crystal rather than at a dislocation. This may beespecially true in the case of p-type or deep level dopants.

Further embodiments of the present invention provide a graphitic and/oramorphous BN passivation layer for wide bandgap semiconductor devices.As used herein, wide bandgap semiconductor devices refers to devicesthat include a semiconductor material having a bandgap of greater thanabout 2.5 eV. Graphitic and/or amorphous BN may be particularly wellsuited for use in GaN based devices because B is isovalent with Al, Gaand In and N is present in both materials. Thus, neither B nor N aredopants in GaN based structures. In contrast, Si is a dopant in GaN.Thus, the formation of a graphitic and/or amorphous BN passivation layermay reduce the likelihood of unintended doping of a GaN layer from Simigration. Furthermore, the graphitic and/or amorphous BN passivationlayer may have reduced trap levels, different trap energies, differentetch selectivity and/or improved annealing behavior as compared toconventional passivation materials, such as SiN or SiO_(x).

Further embodiments of the present invention provide a SiC passivationlayer for Group III-nitride devices. The SiC passivation layer may havereduced trap levels, different trap energies, different etch selectivityand/or improved annealing behavior as compared to conventionalpassivation materials, such as SiN or SiO_(x). References to SiN, SiON,SiO_(x), MgN and the like refer to stoichiometric and/ornon-stoichiometric materials.

Exemplary devices according to some embodiments of the present inventionare schematically illustrated in FIGS. 1A through 12. Thus, whileembodiments of the present invention are described herein with referenceto a recessed gate structure or a non-recessed gate structure, otherembodiments of the present invention may include or not include a gaterecess. Accordingly, embodiments of the present invention should not beconstrued as limited to the particular exemplary embodiments describedherein but may include any suitable structure having a cap layer and/orpassivation layer as described herein.

Turning to FIGS. 1A and 1B, a substrate 10 is provided on which nitridebased devices may be formed. In particular embodiments of the presentinvention, the substrate 10 may be a semi-insulating silicon carbide(SiC) substrate that may be, for example, 4H polytype of siliconcarbide. Other silicon carbide candidate polytypes include the 3C, 6H,and 15R polytypes. The term “semi-insulating” is used descriptivelyrather than in an absolute sense. In particular embodiments of thepresent invention, the silicon carbide bulk crystal has a resistivityequal to or higher than about 1×10⁵ Ω-cm at room temperature.

Optional buffer, nucleation and/or transition layers (not shown) may beprovided on the substrate 10. For example, an AlN buffer layer may beprovided to provide an appropriate crystal structure transition betweenthe silicon carbide substrate and the remainder of the device.Additionally, strain balancing transition layer(s) may also be providedas described, for example, in commonly assigned U.S. Patent PublicationNo. 2003/0102482A1, filed Jul. 19, 2002 and published Jun. 5, 2003, andentitled “STRAIN BALANCED NITRIDE HETEROJUNCTION TRANSISTORS AND METHODSOF FABRICATING STRAIN BALANCED NITRIDE HETEROJUNCTION TRANSISTORS,” orUnited States Patent Publication No. 2004/0012015 A1, filed Jul. 19,2002 and published Jan. 22, 2004, and entitled “STRAIN COMPENSATEDSEMICONDUCTOR STRUCTURES AND METHODS OF FABRICATING STRAIN COMPENSATEDSEMICONDUCTOR STRUCTURES,” the disclosures of which are incorporatedherein by reference as if set forth fully herein.

Appropriate SiC substrates are manufactured by, for example, Cree, Inc.,of Durham, N.C., the assignee of the present invention, and methods forproducing are described, for example, in U.S. Pat. Nos. Re. 34,861;4,946,547; 5,200,022; and 6,218,680, the contents of which areincorporated herein by reference in their entirety. Similarly,techniques for epitaxial growth of Group III nitrides have beendescribed in, for example, U.S. Pat. Nos. 5,210,051; 5,393,993;5,523,589; and 5,592,501, the contents of which are also incorporatedherein by reference in their entirety.

Although silicon carbide may be used as a substrate material,embodiments of the present invention may utilize any suitable substrate,such as sapphire, aluminum nitride, aluminum gallium nitride, galliumnitride, silicon, GaAs, LGO, ZnO, LAO, InP and the like. In someembodiments, an appropriate buffer layer also may be formed.

Returning to FIGS. 1A and 1B, a channel layer 20 is provided on thesubstrate 10. The channel layer 20 may be deposited on the substrate 10using buffer layers, transition layers, and/or nucleation layers asdescribed above. The channel layer 20 may be under compressive strain.Furthermore, the channel layer and/or buffer nucleation and/ortransition layers may be deposited by MOCVD or by other techniques knownto those of skill in the art, such as MBE or HVPE.

In some embodiments of the present invention, the channel layer 20 is aGroup III-nitride, such as Al_(x)Ga_(1-x)N where 0≦x<1, provided thatthe energy of the conduction band edge of the channel layer 20 is lessthan the energy of the conduction band edge of the barrier layer 22 atthe interface between the channel and barrier layers. In certainembodiments of the present invention, x=0, indicating that the channellayer 20 is GaN. The channel layer 20 may also be other GroupIII-nitrides such as InGaN, AlInGaN or the like. The channel layer 20may be undoped (“unintentionally doped”) and may be grown to a thicknessof greater than about 20 Å. The channel layer 20 may also be amulti-layer structure, such as a superlattice or combinations of GaN,AlGaN or the like.

A barrier layer 22 is provided on the channel layer 20. The channellayer 20 may have a bandgap that is less than the bandgap of the barrierlayer 22 and the channel layer 20 may also have a larger electronaffinity than the barrier layer 22. The barrier layer 22 may bedeposited on the channel layer 20. In certain embodiments of the presentinvention, the barrier layer 22 is AlN, AlInN, AlGaN or AlInGaN with athickness of between about 0.1 nm and about 40 nm. Examples of layersaccording to certain embodiments of the present invention are describedin U.S. Patent Publication No. 2002/0167023A1, to Smorchkova et al.,entitled “GROUP-III NITRIDE BASED HIGH ELECTRON MOBILITY TRANSISTOR(HEMT) WITH BARRIER/SPACER LAYER” the disclosure of which isincorporated herein by reference as if set forth fully herein. Inparticular embodiments of the present invention, the barrier layer 22 isthick enough and has a high enough Al composition and doping to induce asignificant carrier concentration at the interface between the channellayer 20 and the barrier layer 22 through polarization effects. Also,the barrier layer 22 should be thick enough to reduce or minimizescattering of electrons in the channel due to ionized impurities orimperfections deposited at the interface between the barrier layer 22and the cap layer 24.

The barrier layer 22 may be a Group III-nitride and has a bandgap largerthan that of the channel layer 20 and a smaller electron affinity thanthe channel layer 20. Accordingly, in certain embodiments of the presentinvention, the barrier layer 22 is AlGaN, AlInGaN and/or AlN orcombinations of layers thereof. The barrier layer 22 may, for example,be from about 0.1 nm to about 40 nm thick, but is not so thick as tocause cracking or substantial defect formation therein. In certainembodiments of the present invention, the barrier layer 22 is undoped ordoped with an n-type dopant to a concentration less than about 10¹⁹cm⁻³. In some embodiments of the present invention, the barrier layer 22is Al_(x)Ga_(1-x)N where 0<x≦1. In particular embodiments, the aluminumconcentration is about 25%. However, in other embodiments of the presentinvention, the barrier layer 22 comprises AlGaN with an aluminumconcentration of between about 5% and about 100%. In specificembodiments of the present invention, the aluminum concentration isgreater than about 10%.

FIG. 1A also illustrates a cap layer 24 on the barrier layer 22 with agate 32 in a recess 36 through the cap layer 24. FIG. 1B alsoillustrates a cap layer 24′ on the barrier layer 22 with a gate 32 onthe cap layer 24′. In some embodiments of the present invention, the caplayer 24, 24′ is a non-uniform composition AlGaN layer. The cap layer24, 24′ moves the top (outer) surface of the device physically away fromthe channel, which may reduce the effect of the surface. The cap layer24, 24′ may be blanket formed on the barrier layer 22 and may beepitaxially grown and/or formed by deposition. Typically, the cap layer24, 24′ may have a thickness of from about 2 nm to about 500 nm.

In some embodiments of the present invention, the cap layer 24, 24′ maybe a graded AlGaN layer. The cap layer 24, 24′ has an outer surface 25that is remote from the barrier layer 22 where the amount of Al in thecap layer 24, 24′ adjacent the surface is greater than an amount of Alin the cap layer 24, 24′ in an internal region of the cap layer 24, 24′.For example, the cap layer 24, 24′ may have a first amount of Al at thesurface 25 and a second amount of aluminum in an internal region of thecap layer 24, 24′ where the first amount is greater than the secondamount. The cap layer 24, 24′ may also have a third amount of Al at theinterface between the cap layer 24, 24′ and the barrier layer 22. Thethird amount may be greater than, less than or equal to the firstamount. In particular embodiments of the present invention, the AlGaNcap layer 24, 24′ includes a first region of Al_(x)Ga_(1-x)N at thesurface 25, where x≦1 and a second region of Al_(y)Ga_(1-y)N in aninternal region of the cap layer 24, 24′, where y<x. In someembodiments, x is from about 0.3 to about 1. In further embodiments, yis from about 0 to about 0.9. In particular embodiments, the AlGaN caplayer includes a third region of Al_(z)Ga_(1-z)N at the interfacebetween the barrier layer 22 and the cap layer 24, 24′, where z≦1 andz≠y. Furthermore, z may be greater than y. For example, in someembodiments of the present invention an AlN layer may be provided as thebarrier layer or a part of the cap layer adjacent the barrier layer. Insuch a case, the cap layer 24, 24′ may include a graded Al concentrationfrom z to y and from y to x. In particular embodiments of the presentinvention having a gate recessed through the cap layer 24, the higherconcentration of Al extends into the cap layer from about 30 Å to about1000 Å. In particular embodiments of the present invention having a gateon the cap layer 24′, the higher concentration of Al extends into thecap layer from about 2.5 Å to about 100 Å.

The cap layer 24, 24′ may be provided by conventional epitaxial growthtechniques where a higher Al concentration is provided duringtermination of growth of the cap layer 24, 24′. Thus, for example, thecap layer 24, 24′ may be provided by MOCVD growth with an increase inthe Al source just prior to and during termination of growth.

As is further illustrated in FIGS. 1A and 1B, ohmic contacts 30 areprovided on the barrier layer 22. A patterned mask and etch process maybe used to expose the underlying barrier layer 22. In some embodimentsof the present invention, the etch may be a low damage etch. In someembodiments of the present invention the etch is a wet etch with astrong base, such as KOH with UV illumination. In other embodiments, theetch is a dry etch. Examples of low damage etch techniques for GroupIII-nitrides include etching techniques other than reactive ion etching,such as inductively coupled plasma using Cl₂, BCl₃, CCl₂F₂ and/or otherchlorinated species or electron cyclotron resonance (ECR) and/ordownstream plasma etching with no DC component to the plasma. As isfurther illustrated in FIGS. 1A and 1B, ohmic metal is patterned toprovide ohmic contact material patterns that when annealed provide theohmic contacts 30. While illustrated as recessed in FIGS. 1A and 1B, insome embodiments of the present invention, the ohmic contacts 30 neednot be recessed.

As illustrated in FIG. 1A, a gate recess may also be provided throughthe cap layer 24 to expose a portion of the barrier layer 22. In someembodiments of the present invention, the recess 36 is formed to extendinto the barrier layer 22. The recess 36 may extend into the barrierlayer 22 to, for example adjust performance characteristics of thedevice such as threshold voltage, frequency performance, etc. The recessmay be formed using a mask and an etch process as described above. Inparticular embodiments where the ohmic contacts 30 provide source anddrain contacts, the recess may be offset between the source and draincontacts such that the recess, and subsequently the gate contact 32, iscloser to the source contact than the drain contact.

A gate contact 32 is formed in the recess and contacts the exposedportion of the barrier layer 22. The gate contact may be a “T” gate asillustrated in FIG. 1A and may be fabricated using conventionalfabrication techniques. The gate contact 32 may also be formed on thecap layer 24′ as illustrated in FIG. 1B and may be fabricated usingconventional fabrication techniques. Suitable gate materials may dependon the composition of the barrier layer, however, in certainembodiments, conventional materials capable of making a Schottky contactto a nitride based semiconductor material may be used, such as Ni, Pt,NiSi_(x), Cu, Pd, Cr, W and/or WSiN.

A conventional passivation layer or a BN passivation layer as describedbelow may also be provided on the structures of FIGS. 1A and 1B. Forexample, a SiN layer and, in some embodiments, an extremely thin SiNlayer, may be formed in situ. A MgN passivation layer, such as thatdescribed in U.S. Pat. No. 6,498,111 entitled “FABRICATION OFSEMICONDUCTOR MATERIALS AND DEVICES WITH CONTROLLED ELECTRICALCONDUCTIVITY,” the disclosure of which is incorporated herein byreference as if set forth in its entirety, could also be utilized.Optionally, an anneal of the structure including the passivation layermay be carried out in an oxygen environment to remove hydrogen from thelayer and alter surface states and/or add oxygen to the surface. If anoxygen anneal is performed, the anneal may be performed in a manner tonot significantly oxidize the layer between the passivation layer andthe underlying Group III-nitride layer. For example, in some embodimentsof the present invention, the annealing may be carried out at atemperature of from about 100° C. to about 1000° C. and for a time offrom about 10 seconds to about 1 hour. The oxygen containing environmentmay be only oxygen, oxygen in N₂, oxygen in another inert gas, such asargon, oxygen in dry air, CO, CO₂, NO, NO₂ and/or ozone. The gases usedto provide the oxygen containing environment may be free of hydrogen soas to not incorporate hydrogen into the passivation layer. Alternativelyor additionally, an anneal may be carried out in D₂ or D₂O.

Transistors according to embodiments of the present invention may befabricated utilizing techniques such as those discussed in the patentapplications and patents incorporated by reference herein, including,for example, as described in U.S. patent application Ser. No.10/849,617, filed May 20, 2004 and entitled “METHODS OF FABRICATINGNITRIDE-BASED TRANSISTORS HAVING REGROWN OHMIC CONTACT REGIONS ANDNITRIDE-BASED TRANSISTORS HAVING REGROWN OHMIC CONTACT REGIONS” and U.S.patent application Ser. No. 10/897,726, filed Jul. 23, 2004 and entitled“METHODS OF FABRICATING NITRIDE-BASED TRANSISTORS WITH A CAP LAYER AND ARECESSED GATE,” the disclosures of which are incorporated herein as ifdescribed in their entirety.

FIGS. 2A and 2B illustrate high electron mobility transistors having acap layer 34, 34′ according to further embodiments of the presentinvention. The substrate 10, channel layer 20, barrier layer 22, ohmiccontacts 30 and gate contact 32 may be provided as discussed above withreference to FIGS. 1A and 1B. As seen in FIGS. 2A and 2B, the cap layer34, 34′ includes a doped region 40 at or near the outer surface of thecap layer 34, 34′. The cap layer 34, 34′ may be a GaN based cap layer,such as a GaN layer and/or an AlGaN layer as described, for example, inthe patents and patent application incorporated by reference herein. Insome embodiments of the present invention, the doped region 40 is dopedwith p-type dopant, such as Mg, Be, Zn, Ca and/or C. In otherembodiments of the present invention, the doped region 40 is doped withan n-type dopant, such as Si, Ge and/or O. In further embodiments of thepresent invention, the doped region 40 is doped with a deep leveldopant, such as Fe, C, V, Cr, Mn, Ni and/or Co. The dopant may beincorporated into the cap layer 34 during deposition or growth of thecap layer 34, 34′ or may be subsequently implanted, for example, usingion implantation. In some embodiments of the present invention, the caplayer 34 has a dopant incorporated throughout the cap layer 34, 34′. Insuch a case, the doped region 40 may be provided by a region ofincreased dopant concentration over the concentration of dopant in theremainder of the cap layer 34, 34′. Techniques for co-doping GroupIII-nitride materials are described, for example, in U.S. patentapplication Ser. No. 10/752,970, filed Jan. 7, 2004 entitled “CO-DOPINGFOR FERMI LEVEL CONTROL IN SEMI-INSULATING GROUP III NITRIDES,” thedisclosure of which is incorporated herein as if set forth in itsentirety.

In embodiments of the present invention where the dopants are n-typedopants, the n-type dopants may be Si, Ge or O. In particularembodiments of the present invention without a gate recess, the dopedregion 40 extends into the cap layer 34 from about 2.5 Å to about 50 Å.In particular embodiments of the present invention with a gate recess,the doped region 40 extends into the cap layer 34′ from about 20 Å toabout 5000 Å. With n-type dopants, the doped region 40 in embodimentswithout a gate recess may provide a dopant concentration of from about10¹⁸ to about 10²¹ cm⁻³ and may be more heavily doped than 10²¹ cm⁻³ ifa gate recess is provided. In particular embodiments of the presentinvention, the doped region 40 may be one or more delta-doped regions ator near the surface of the cap layer 34, 34′ and may, for example, havea dopant concentration of from about 10¹¹ to about 10¹⁵ cm⁻². As usedherein, a delta-doped region is at the surface if it is within about 5 Åof the surface and near the surface if it is within about 50 Å of thesurface. In particular embodiments of the present invention, the dopantis O that extends into the cap layer 34, 34′ about 20 Å. N-type dopantsmay be used to screen the channel region from surface states and pin thesurface energy level at a predictable and desired level to reduce and/orminimize trapping effects. The level of doping should be sufficientlyhigh so as to be the dominant “surface” state in embodiments without arecessed gate but not so high as to provide excessive current leakagepaths.

In other embodiments, the doped region 40 is a region doped with p-typedopants. In particular embodiments of the present invention without agate recess, the doped region 40 extends into the cap layer 34 fromabout 2.5 Å to about 100 Å. In particular embodiments of the presentinvention with a gate recess, the doped region 40 extends into the caplayer 34′ from about 30 Å to about 5000 Å. With p-type dopants, thedoped region 40 may provide a dopant concentration of from about 10¹⁶ toabout 10²² cm⁻³. The p-type dopants may be Mg, Be, Zn, Ca and/or C. Inparticular embodiments of the present invention, the doped region may beone or more delta-doped regions at or near the surface of the cap layerand may, for example, have a dopant concentration of from about 10¹¹ toabout 10¹⁵ cm⁻². P-type dopants may be used to screen the channel regionfrom surface states, pin the surface energy level at a predictable anddesired level to reduce and/or minimize trapping effects and to reduceleakage currents. The level of doping should be sufficiently high so asto reduce leakage current in embodiments without a recessed gate and bethe dominant “surface” state but not so high as to provide introducetraps or leakage paths by becoming a conductive layer. However, inparticular embodiments of the present invention have a recessed gate asillustrated, for example, in FIG. 2B, if an insulating region, such as aSiN layer or gap, is provided between the cap layer 34′ and the gatecontact 32, a high level of p-type dopants may be provided such that thecap layer 34′ may be provided as a conductive layer.

Furthermore, in certain embodiments of the present invention, the dopedregion 40 may be doped with p-type dopants so as to provide a p-njunction between the doped region and the cap layer 34 and the gatecontact 32 is provided directly on the doped region 40 so as to providea Junction HEMT (JHEMT). In such a case, the dope region 40 would notextend to the ohmic contacts 30, which may be isolated from the dopedregion by an insulating region, such as a SiN layer or gap.

In still further embodiments, the doped region 40 is a region doped withdeep level dopants. In particular embodiments of the present inventionwithout a gate recess, the doped region 40 extends into the cap layer 34from about 2.5 Å to about 100 Å. In particular embodiments of thepresent invention with a gate recess, the doped region 40 extends intothe cap layer 34′ from about 30 Å to about 5000 Å. With deep leveldopants, the doped region 40 may provide a dopant concentration of fromabout 10¹⁶ to about 10²² cm⁻³. The deep level dopants may be Fe, C, V,Cr, Mn, Ni, Co or other rare earth elements. Deep level dopants may beused to screen the channel region from surface states, pin the surfaceenergy level at a predictable and desired level to reduce and/orminimize trapping effects and to reduce leakage currents. The level ofdoping should be sufficiently high so as to reduce leakage current inembodiments without a recessed gate and be the dominant “surface” statebut not so high as to cause significant trapping.

FIGS. 3A and 3B illustrate electronic devices incorporating a graphiticand/or amorphous BN passivation layer according to some embodiments ofthe present invention. The substrate 10, channel layer 20, barrier layer22, cap layer 24, ohmic contacts 30 and gate contact 32 may be providedas discussed above with reference to FIGS. 1A, 1B and/or FIGS. 2A, 2B.As is further illustrated in FIGS. 3A and 3B, a graphitic and/oramorphous BN passivation layer 100, 100′ is provided on exposed surfacesof the device. In particular embodiments of the present invention, thegraphitic BN passivation layer 100, 100′ is a non-single crystal layer.The graphitic and/or amorphous BN passivation layer 100, 100′ may beprovided as a single layer or may be multiple layers and may beincorporated with layers of other materials, such as SiN or SiO_(x). Inparticular embodiments of the present invention the graphitic oramorphous BN passivation layer 100 where the gate is recessed throughthe BN passivation layer 100, the BN passivation layer 100 may have athickness of from about 3 Å to about 1 μm. In particular embodiments ofthe present invention the graphitic or amorphous BN passivation layer100′ where the gate is not recessed through the BN passivation layer100′, the BN passivation layer 100′ may have a thickness of from about 2Å to about 100 Å. Thus, in the embodiments illustrated in FIG. 3B, aMISHEMT may be provided. Furthermore, as discussed above, the gate maybe recessed into or through the cap layer 24 as illustrated, forexample, in FIGS. 1A and 2B, and the BN passivation layer 100, 100′ mayextend into the recess in the cap layer 24, into the recess and onto thebarrier layer 22 or may terminate at the gate contact 32. Thus, in someembodiments of the present invention a MISHEMT may be provided with arecessed gate.

Techniques for forming graphitic and/or amorphous BN, such as by MOCVD,are known to those of skill in the art and, therefore, need not bedescribed further herein. For example, a BN layer may be formed byflowing TEB and NH₃ in a carrier gas. However, the formation of thegraphitic and/or amorphous BN passivation layer 100 should be carriedout at temperatures below the decomposition temperature of theunderlying structure on which the passivation layer 100 is formed. Thus,for example, for a GaN based structure, the graphitic and/or amorphousBN passivation layer 100 should be formed at temperatures of less thanabout 1100° C. and in some embodiments less than about 950° C. In someembodiments, the passivation layer 100 may be subsequently annealed asdescribed above.

FIGS. 4A and 4B illustrate electronic devices incorporating a SiCpassivation layer according to some embodiments of the presentinvention. The substrate 10, channel layer 20, barrier layer 22, caplayer 24, ohmic contacts 30 and gate contact 32 may be provided asdiscussed above with reference to FIGS. 1A, 1B and/or FIGS. 2A, 2B. Asis further illustrated in FIGS. 4A and 4B, a SiC passivation layer 110,110′ is provided on exposed surfaces of the device. In particularembodiments of the present invention, the SiC passivation layer 110,110′ is a non-single crystal layer. In some embodiments of the presentinvention, the SiC passivation layer 110, 110′ is insulating or p-typeSiC. If the SiC passivation layer, 110′ is p-type SiC, an insulatingregion, such as a SiN layer or a gap, may be provided between the SiCpassivation layer 110, 110′ and the ohmic contacts 32. In someembodiments of the present invention, the SiC passivation layer 110,110′ is 3C SiC as 3C SiC may be formed on on-axis (0001) hexagonalmaterials in a low temperature process. The SiC passivation layer 110,110′ may be provided as a single layer or may be multiple layers and maybe incorporated with layers of other materials, such as SiN or SiO₂. Inparticular embodiments of the present invention the SiC passivationlayer 110 where the gate is recessed through the SiC passivation layer110, the SiC passivation layer 110 may have a thickness of from about 3Å to about 1 μm. In particular embodiments of the present invention theSiC passivation layer 110′ where the gate is not recessed through theSiC passivation layer 110′, the SiC passivation layer 110′ may have athickness of from about 2 Å to about 100 Å. Thus, in the embodimentsillustrated in FIG. 4B, a MISHEMT may be provided. Furthermore, asdiscussed above, the gate may be recessed into or through the cap layer24 as illustrated, for example, in FIGS. 1A and 2B, and the SiCpassivation layer 110, 110′ may extend into the recess in the cap layer24, into the recess and onto the barrier layer 22 or may terminate atthe gate contact 32. Thus, in some embodiments of the present inventiona MISHEMT may be provided with a recessed gate.

Techniques for forming SiC layers are known to those of skill in the artand, therefore, need not be described further herein. However, theformation of the SiC passivation layer 110 should be carried out attemperatures below the decomposition temperature of the underlyingstructure on which the passivation layer 110 is formed. Thus, forexample, for a GaN based structure, the SiC passivation layer 110 shouldbe formed at temperatures of less than about 1100° C. and in someembodiments less than about 950° C. Techniques for forming SiC at suchlow temperatures may include CVD or PECVD using, for example, SiH₄ andC₃H₈ as Si and C sources, or very low temperature sputtering.Furthermore, the SiC layer may be doped with impurities to control theproperties of the SiC passivation layer 110. For example, n-type SiC maybe doped with N, p-type SiC may be doped with Al and/or B and insulatingSiC may be doped with V or Fe. In some embodiments, the passivationlayer 100 may be subsequently annealed as described above.

While FIGS. 3A, 3B and 4A, 4B illustrate passivation layers 100, 100′and 110, 110′ on a cap layer 24, other cap layers, such as the cap layer34, conventional single or multiple cap layers or no cap layer may beprovided. For example, the passivation layers 100, 100′ and 110, 110′could be used with a cap layer that included an AlN layer at its outersurface such that the passivation layers were provided on the AlN layer.Thus, the use of a graphitic or amorphous BN passivation layer 100, 100′or a SiC passivation layer 110, 110′ should not be construed as limitedto the particular structure illustrated in FIGS. 3A, 3B and 4A, 4B butmay be used on any Group III-nitride semiconductor device or other widebandgap semiconductor device.

While embodiments of the present have been described with reference toHEMT structures where the gate is directly on the barrier or cap layers,in some embodiments of the present invention an insulating layer may beprovided between the gate and the barrier or cap layer. Thus, in someembodiments of the present invention, an insulating gate HEMT may beprovided, for example, as described in U.S. Patent Publication No.2003/0020092 entitled “INSULATING GATE ALGAN/GAN HEMT”, to Parikh etal., the disclosure of which is incorporated herein by reference as ifset forth fully herein. In some embodiments, the insulating layer may beof graphitic and/or amorphous BN.

FIGS. 5A and 5B illustrate further embodiments of the present inventionincorporating an AlN cap layer 54, 54′. FIG. 5A also illustrates an AlNcap layer 54 on the barrier layer 22 with a recessed gate 32 through theAlN cap layer 54. FIG. 5B also illustrates an AlN cap layer 54′ on thebarrier layer 22 with a gate 32 on the AlN cap layer 54′. The cap layer54, 54′ moves the top (outer) surface of the device physically away fromthe channel, which may reduce the effect of the surface. Furthermore,the AlN cap layer 54, 54′ may provide increased chemical stability andprotect the underlying layers in that the AlN cap layer 54, 54′ may notbe susceptible to etching or other chemical reactions at hightemperatures due to the stronger Al—N bonds compared to Ga—N bonds.

The AlN cap layer 54, 54′ may be blanket formed on the barrier layer 22and may be epitaxially grown and/or formed by deposition. Typically, thecap layer 54, 54′ may have a thickness of from about 0.2 nm to about 500nm. In particular embodiments of the present invention having a gaterecessed through the AlN cap layer 54, the AlN cap layer 54 has athickness of from about 10 Å to about 5000 Å. In particular embodimentsof the present invention having a gate on the AlN cap layer 54′, the AlNcap layer 54′ has a thickness of from about 2 Å to about 50 Å.

The AlN cap layer 54, 54′ may be provided by conventional epitaxialgrowth techniques by termination of the Ga source during termination ofgrowth of the barrier layer 22. Thus, for example, the AlN cap layer 54,54′ may be provided by MOCVD growth by termination of the Ga source justprior to and during termination of growth.

FIG. 6 illustrates further embodiments of the present invention where aprotective layer 64 is provided on the barrier layer 22. As isillustrated in FIG. 6, the ohmic contacts are provided on the protectivelayer 64. The gate contact 32 may also be provided on the protectivelayer 64. In certain embodiments of the present invention, the ohmiccontacts 30 are provided directly on the protective layer 64 and thegate contact 32 may also be provided directly on the protective layer64.

The protective layer 64 may be a SiN layer deposited prior to formationof the ohmic contacts 30 and the gate contact 32. Alternatively, theprotective layer 64 may be a BN or MgN layer. MgN may be especiallysuitable for use with p-type devices as additional doping may beprovided upon anneal of the ohmic contact material. The protective layer64 may be a single layer, such as a single SiN, MgN or BN layer, or, insome embodiments, protective layer 64 may be provided as multiplelayers, such as a layer of SiN and a layer of AlN.

The protective layer 64 may have a thickness of from about 1 Å to about10 Å and, in some embodiments, may have a thickness of about onemonolayer. Because the protective layer 64 is thin, there may be no needto recess the ohmic contacts through the protective layer 64.Reliability may be improved through better surface state control andlower gate leakage current in comparison to devices without such aprotective layer.

The protective layer 64 may be formed in-situ with the formation of thebarrier layer. Because the protective layer 64 is very thin, there maybe very little additional fabrication cost other than providing a Sisource, B source or Mg source and only a small additional growth time todeposit the thin protective layer 64. Furthermore, because theprotective layer 64 is thin, no additional steps may be needed to formrecesses for the gate and/or ohmic contacts.

In the fabrication processes discussed above, gates may be formed byetching a 0.3 to 0.7 μm line in a layer of SiN that has a thickness offrom about 50 nm to about 150 nm, forming a longer line, for example,1.0 μm or more, overlapping the previous line by from about 0.1 to 0.6μm on each side, in a photoresist and depositing metal into the line.The excess metal may be removed by dissolving the photoresist.

While the gate is etched it may be possible to inadvertently etch theAlGaN and/or hot electrons striking the AlGaN/SiN interface, especiallyon the drain side of the gate, may create trap states at the interfaceor in the SiN, which may cause the device to be unreliable.

Thus, according to some embodiments of the present invention a thin AlNlayer may be provided on an AlGaN cap layer, which may increase theselectivity of the etch, reducing the amount of AlGaN that is etched. Asused herein, a “thin AlN layer” refers to an AlN layer having athickness of from about a few monolayers to about 50 Å. Furthermore, AlNhas a higher bandgap and stronger bonding properties relative to AlGaN,so the amount of electron injection into the SiN interface or the SiNitself may be reduced. It will be understood that according to someembodiments of the present invention the design of the epitaxial layershould be done to take polarization fields into account in order toaccomplish the reduction of electron injection.

According to some embodiments of the present invention including a thinAlN or high mole-fraction AlGaN layer, the etch stopping qualities ofthe AlN may be combined with the highest possible potential barrier asdiscussed further below.

Referring now to FIG. 7, further embodiments of the present inventioninclude a multilayer cap layer (65, 66) and a protective layer 67 on thebarrier layer 22. As is illustrated in FIG. 7, the multilayer cap layerincludes a layer including AlN 65 and a layer of GaN 66. The layerincluding AlN 65 may include AlGaN, AlInd and/or AlN. The layerincluding AlN 65 may have a thickness of from about 3.0 to about 30.0 Å.In some embodiments of the present invention, the layer including AlN 65may be an AlGaN layer having a high mole-fraction. As used herein, “molefraction” refers to the AlN mole-fraction in an AlN—GaN alloy. As usedherein, “a high mole-fraction” refers to a mole fraction of from about30 to about 100%.

In some embodiments of the present invention, the GaN layer 66 may bereplaced by, for example, an AlGaN layer having a low mole-fraction. Asused herein, “a low mole-fraction” refers to a mole fraction of fromabout 0 to about 30%. The mole-fraction of the AlGaN layer may be fromabout zero percent of a mole-fraction of the barrier layer to about themole-fraction of the barrier layer. The mole-fraction of the AlGaN layermay be greater than the mole-fraction of the barrier layer.

It will be understood that although the gate 32 illustrated inembodiments of the present invention illustrated in FIG. 7 is notrecessed into the multilayer cap layer, embodiments of the presentinvention are not limited to the configuration illustrated therein. Forexample, the gate 32 may be recessed without departing from the scope ofthe present invention.

Referring now to FIG. 8, further embodiments of the present inventioninclude a multilayer cap layer (73, 74) on the barrier layer 22. As isillustrated in FIG. 8, the multilayer cap layer includes an AlGaN 73layer on the barrier layer 22 and a layer of AlN 74 on the AlGaN layer73. In embodiments of the present invention illustrated in FIG. 8, theAlN layer 74 may have a thickness of about 10 Å. As discussed above, theAlN layer 74 may include an AlGaN layer having a high mole-fraction.

As further illustrated in FIG. 8, the gate contact 32 is not recessedinto the multilayer cap layer. In these embodiments of the presentinvention, a thickness of the AlGaN layer 73 may be from about 5.0 toabout 50.0 Å. It will be understood that embodiments of the presentinvention are not limited to the configuration illustrated in FIG. 8.For example, if the gate contact 32 were recessed the thickened of theAlGaN layer 73 may be much larger, such as about 250 Å. Simulatedconduction band edge and electron density as a function of depth fordevices according to embodiments of the present invention illustrated inFIG. 8 will be discussed below with respect to FIG. 11.

Referring now to FIG. 9, further embodiments of the present inventioninclude a multilayer cap layer (73′, 74′, 75) on the barrier layer 22.As is illustrated in FIG. 9, the multilayer cap layer includes a layerincluding AlGaN 73′ on the barrier layer 22, a layer of AlN 74′ on theAlGaN layer 73′ and a layer of GaN 75 on the AlN layer 74′. Inembodiments of the present invention illustrated in FIG. 9, the AlNlayer 74′ has a thickness of about 10.0 Å and the GaN layer has athickness of about 20 Å. Embodiments of HEMTs illustrated in FIG. 9 mayhave a barrier of greater than about 3.0 V.

As illustrated in FIG. 9, the gate contact 32 is not recessed into themultilayer cap layer. Although, it will be understood that embodimentsof the present invention are not limited to the configurationillustrated in FIG. 9. Simulated conduction band edge and electrondensity as a function of depth for devices according to embodiments ofthe present invention illustrated in FIG. 9 will be discussed below withrespect to FIG. 12.

Referring now to the graphs illustrated in FIGS. 10 through 12. Inparticular, FIG. 10 illustrates simulated conduction band edge andelectron density as a function of depth for devices including an AlGaNcap and no AlN or GaN cap. FIG. 11 illustrates simulated conduction bandedge and electron density as a function of depth for devices accordingto embodiments of the present invention illustrated in FIG. 8. Asillustrated in FIG. 11, the addition of the AlN layer 74, a widerbandgap material, may induce an excessive polarization charge, thuslowering the effective barrier of the device. FIG. 12 illustratessimulated conduction band edge and electron density as a function ofdepth for devices according to embodiments of the present inventionillustrated in FIG. 9. As illustrated in FIG. 12, a multilayer capincluding AlN with a GaN cap on the AlN may result in a relativelyhigher barrier (over 3.0 V vs. 1.25 for the standard structure) with aroughly similar amount of charge in the channel as illustrated in FIG.10. It will be understood that in some embodiments of the presentinvention, the thicknesses of the GaN layer 75 and the AlN layer 74′ maybe changed to tune the resulting charge density.

Referring to the graphs of FIGS. 10 through 12 illustrating simulatedconduction band edge and electron density as a function of depthaccording to some embodiments of the present invention, a constant Ecfor the top interface was used. In some embodiments of the presentinvention, the conduction band edge of the AlGaN at the top of astandard structure may be higher than the conduction band edge of GaNand the conduction band edge of AlN may be even higher.

The thickness of the layers of the multilayer cap (GaN, AlN, and AlGaN)and the composition of the AlGaN can all be adjusted to obtain theimproved device performance. Furthermore, as discussed above, in someembodiments of the present invention, the GaN layer may be replaced bylow mole-fraction AlGaN. The mole-fraction of the AlGaN may be anywherebetween 0% to the same fraction as the main AlGaN barrier layer, orpossibly even higher in some embodiments.

Furthermore, in some embodiments of the present invention the AlN layermay be replaced by high mole-fraction AlGaN. This may compromise theetch stop, plus the use of a very thin barrier of only a few monolayersmay be compromised in the sense that the barrier height may be lower inthe immediate vicinity of a Ga site.

While embodiments of the present invention have been described hereinwith reference to particular HEMT structures, the present inventionshould not be construed as limited to such structures. For example,additional layers may be included in the HEMT device while stillbenefiting from the teachings of the present invention. Such additionallayers may include GaN cap layers, as for example, described in Yu etal., “Schottky barrier engineering in III-V nitrides via thepiezoelectric effect,” Applied Physics Letters, Vol. 73, No. 13, 1998,or in U.S. Patent Publication No. 2002/0066908A1 filed Jul. 12, 2001 andpublished Jun. 6, 2002, for “ALUMINUM GALLIUM NITRIDE/GALLIUM NITRIDEHIGH ELECTRON MOBILITY TRANSISTORS HAVING A GATE CONTACT ON A GALLIUMNITRIDE BASED CAP SEGMENT AND METHODS OF FABRICATING SAME,” thedisclosures of which are incorporated herein by reference as if setforth fully herein. In some embodiments, insulating layers such as SiN,an ONO structure or relatively high quality AlN may be deposited formaking a MISHEMT and/or passivating the surface. The additional layersmay also include a compositionally graded transition layer or layers.

Furthermore, the barrier layer 22 may also be provided with multiplelayers as described in U.S. Patent Publication No. 2002/0167023A1 citedabove. Thus, embodiments of the present invention should not beconstrued as limiting the barrier layer to a single layer but mayinclude, for example, barrier layers having combinations of GaN, AlGaNand/or AlN layers. For example, a GaN, AlN structure may be utilized toreduce or prevent alloy scattering. Thus, embodiments of the presentinvention may include nitride based barrier layers, such nitride basedbarrier layers may include AlGaN based barrier layers, AlN based barrierlayers and combinations thereof.

While embodiments of the present invention have been described withreference to the ohmic contacts 30 being recessed through the variouscap layers, in certain embodiments of the present invention, the ohmiccontacts 30 are provided on the cap layer or only partially recessedinto the cap layer. Thus, embodiments of the present invention shouldnot be construed as limited to structures having ohmic contacts recessedthrough the cap layer.

In the drawings and specification, there have been disclosed typicalembodiments of the invention, and, although specific terms have beenemployed, they have been used in a generic and descriptive sense onlyand not for purposes of limitation.

1. A Group III-nitride high electron mobility transistor (HEMT),comprising: a Group III-nitride based channel layer; a Group III-nitridebased barrier layer on the channel layer; a multilayer cap layer on thebarrier layer, the multilayer cap layer including a layer includingaluminum nitride (AlN) on the barrier layer and a gallium nitride (GaN)layer on the layer including AlN; and a SiN passivation layer on the GaNlayer.
 2. The HEMT of claim 1, wherein the layer including AlN comprisesAluminum gallium nitride (AlGaN), aluminum indium nitride (AlInN) and/orAlN.
 3. The HEMT of claim 1, wherein the layer including AlN has athickness of from about 3.0 to about 30.0 Å.
 4. The HFMT of claim 1,wherein the GaN layer comprises an aluminum gallium nitride (AlGaN)layer having a low mole-fraction.
 5. The HEMT of claim 4, wherein themole-fraction of the AlGaN layer is from about zero percent of amole-fraction of the barrier layer to about the mole-fraction of thebarrier layer.
 6. The HEMT of claim 5, wherein the mole-fraction of theAlGaN layer is greater than the mole-fraction of the barrier layer. 7.The HEMT of claim 1, wherein the layer including AlN comprises analuminum gallium nitride (AlGaN) layer having a high mole-fraction.
 8. AGroup III-nitride high electron mobility transistor (HEMT), comprising:a Group III-nitride based channel layer; a Group III-nitride basedbarrier layer on the channel layer; and a multilayer cap layer on thebarrier layer, the multilayer cap layer including an aluminum galliumnitride (AlGaN) layer on the barrier layer and an aluminum nitride (AlN)layer on the AlGaN layer.
 9. The HEMT of claim 8, wherein the AlN layerhas a thickness of about 10 Å.
 10. The HEMT of claim 9, furthercomprising a gate contact that is not recessed into the multilayer caplayer and wherein a thickness of the AlGaN layer is from about 5.0 toabout 50.0 Å.
 11. The HEMT of claim 8, wherein the AlN layer comprisesan AlGaN layer having a high mole-fraction.
 12. A Group III-nitride highelectron mobility transistor (HEMT), comprising: a Group III-nitridebased channel layer; a Group III-nitride based barrier layer on thechannel layer; and a multilayer cap layer on the barrier layer, themultilayer cap layer including an aluminum gallium nitride (AlGaN) layeron the barrier layer, an aluminum nitride (AlN) layer on the AlGaN layerand a gallium nitride (GaN) layer on the AlN layer.
 13. The HEMT ofclaim 12, wherein the AlN layer has a thickness of about 10.0 Å and theGaN layer has a thickness of about 20 Å.
 14. The HEMT of claim 12,wherein the HEMT has a barrier of greater than about 3.0 V.
 15. The HEMTof claim 12, wherein the GaN layer comprises an AlGaN layer having a lowmole-fraction.
 16. The HEMT of claim 15, wherein the mole-fraction ofthe AlGaN layer is from about zero percent of a mole-fraction of thebarrier layer to about the mole-fraction of the barrier layer.
 17. TheHEMT of claim 16, wherein the mole-fraction of the AlGaN layer isgreater than the mole-fraction of the barrier layer.
 18. The HEMT ofclaim 12, wherein the AlN layer comprises an AlGaN layer having a highmole-fraction.